The present invention relates to a rubber composition used for a sidewall of an automobile tire and a pneumatic tire using the same.
In order to reduce rolling resistance of a tire, a tire can be made lightweight. The conventional method for obtaining a lightweight tire is the method of reducing the amount of rubber that forms the tire. However, durability of the tire is decreased by this method. Also, because a small amount of rubber is used, stiffness of the tire and steering stability tend to decrease. When the amount of the filler in a rubber composition is reduced, rolling resistance can be reduced, but reinforcement properties of the rubber decrease.
It is known that both improvement in fatigue of the rubber and reduced heat generation can be obtained, when a thermoplastic elastomer or an olefin resin is compounded with the rubber component as a reinforcing agent of fiber (see JP-A-7-309974,
JP-A-7-309975). It is also known that rubber having high elasticity and high stiffness can be obtained and destruction properties can be improved, by compounding with polyolefins such as polyethylene (see JP-A-9-272307, JP-A-10-315717, JP-A-10-195249). However, in these methods, the kneading temperature of the rubber composition is high and exceeds the melting point of the polyolefin. As a result, dispersion of the compounded polyolefin is insufficient. Consequently, the steering stability is insufficient and a lightweight tire cannot be prepared. In addition, the polyolefin powder is poor in adhesion with the diene rubber which is known to easily become the destruction point.
Also, known is the method of burying a thermoplastic elastomer composition in a sidewall, in order to reduce the road noise of a tire (see JP-A-9-300921). However, according to this method, the rolling resistance cannot be reduced while maintaining the steering stability.